1. Field of the Invention
The present invention relates to a phosphor for a cathode ray tube. More particularly, the present invention relates to a phosphor for a cathode ray tube having excellent luminance brightness, chromaticity and proper afterglow.
2. Description of the Prior Art
Recently, a cathode ray tube for display indication is frequently employed for the control or monitoring of a terminal equipment of a computer or a plant system. In such a case, the display indication is a numeral, alphabet, katakana i.e. the square Japanese syllabary or figure. For this purpose, a color cathode ray tube is advantageous because it functionally has a great capacity for informations and the display indication can readily be read. However, a usual cathode ray tube is designed to display mainly continuously moving pictures, and therefore, it is desired that afterglows of the respective phosphors emitting different colors are short enough so that they are invisible. Whereas, the above-mentioned cathode ray tube for display indication is primarily intended to display letters, characters or figures which are still for a certain period of time. Accordingly, by employing a phosphor showing suitable afterglows, it is possible to reduce the flickering of the image so that the image can clearly be seen, and it is also possible to reduce the number of picture elements to be transmitted. This gives an observer less eye strain, and chances of misreading will be reduced. Further, the circuit can advantageously be simplified.
As a representative phosphor currently employed for such a purpose, there may be mentioned a green emitting Zn.sub.2 SiO.sub.4 :Mn,As phosphor or a red emitting (Zn,Mg).sub.3 (PO.sub.4).sub.2 :Mn phosphor. The former has B.sub.10% (reduction time of luminance brightness to 10% after stopping of excitation) of 150 msec. and the latter has B.sub.10% of 120 msec. Thus, they have satisfactory afterglow characteristics.
However, the above-mentioned conventional long-afterglow phosphors generally show somewhat poor emission efficiency. Particularly, as the spectral luminous efficacy of the red phosphor is low, its luminance brightness is insufficient as compared with the green phosphor. In order to supplement the want of luminance brightness, a short afterglow-red phosphor having high luminance brightness used for a color cathode ray tube for a television, for instance, Y.sub.2 O.sub.2 S:Eu, is incorporated (Japanese Patent No. 917125). However, as its afterglow is as short as B.sub.10% =2 msec., there is a problem that the amount of incorporation is limited.